Vessel formation from mesodermal precursors, vasculogenesis, is regulated by a complex interplay of positive and negative factors acting on endothelial cells. Overlapping morphogenetic events include: i) establishment of the endothelial lineage, ii) positioning and patterning of primordial endothelial cells, iii) formation of lumens, iv) establishment of a primary polygonal network, and v) elaboration of larger vessels. At stages before circulation begins, large caliber vessels, such as the aortae, form when the lumens of neighboring primary tubes coalesce. We use the term vascular fusion to describe the mechanism by which small caliber tubes fuse so as to form large caliber vessels. We previously showed that formation of the aorta is regulated, at least partially, by VEGF signaling and integrin-mediated adhesion. Neutralization of integrin alphavbeta3 activity prevents vascular fusion, while VEGF overstimulation promotes vascular fusion. The hypothesis to be tested is that precise local interactions between VEGF and specific integrins regulate endothelial cell motility and shape changes resulting in the degree of vascular fusion or the absence of vascular fusion. These vasculogenic events take place in ten hours during the second day of quail gestation. The specific aims of this work are: 1) to analyze VEGFR-1 versus VEGFR-2 mediated signaling in primordial endothelial cells, 2) to examine the role integrins alphavbeta3 and alpha5beta1 play in vascular fusion, and 3) to establish computational bioassays with which to quantify individual and collective endothelial cell behaviors. Primordial endothelial cells will be examined using time-lapse microscopy and specific markers in avian embryos. Convincing quantitative data will be produced that describe endothelial velocity, trajectory and persistence time of motility during tube formation, in vivo. The long-term objective is to compile a database for future computer modeling of vasculogenesis. Collectively, this work will contribute to our knowledge of embryonic and adult neovascular processes. Understanding vessel development is of overwhelming clinical importance. Our goal and the goal of other vascular biologists is to explain the mechanisms that promote or inhibit vessel growth. The dynamic embryonic environment provides an ideal opportunity to examine vascular morphogenesis in precise detail.